Parallel plate structure provided with PZT thin-film bimorph and method of fabrication thereof

ABSTRACT

A parallel plate structure ( 1 ) is provided with a pair of bimorph piezoelectric elements ( 2 ) and prismatic insulation spacers ( 3 ) inserted between the piezoelectric elements ( 2 ) at the upper and lower ends thereof for cementing the piezoelectric elements ( 2 ) together via the spacers ( 3 ). Each piezoelectric element ( 2 ) comprises a planar base material ( 4 ) of titanium, PZT thin films ( 5 ) formed on both sides of the base material ( 4 ) by the hydrothermal method, and electrode films ( 6 ) formed on the PZT thin films ( 5 ). The base material ( 4 ) is 20 μm thick and the PZT thin films ( 5 ) are several pm thick, while the aluminum electrode films ( 6 ) are several um thick.

BACKGROUND OF THE INVENTION

The present invention relates to a parallel plate structure providedwith a PZT lead (Pb) Zirconate Titanate) thin-film bimorph and method offabrication thereof, and more particularly, to structures used aspiezoelectric actuators.

A bimorph, which is known in the prior art, includes a plate-likesubstrate, two PZT devices (lead zirconate titanate: ceramics containinga solid solution of lead titanate and lead zirconate), which are formedon the front and rear surfaces of the substrate and which function aspiezoelectric devices, and electrodes formed on each PZT device. Voltageis applied to both of the PZT devices so that one of the PZT devicesexpands while the other contracts. Since this deforms the bimorphentirely in a certain direction, the bimorph is used as an actuator.

However, when employing preformed PZT devices, it is difficult to makethe devices thinner in subsequent processes. Accordingly, the productionof a more compact bimorph is difficult. Furthermore, alot of time isrequiring to adhere the PZT devices to the front and rear surfaces thesubstrate. This lengthens the fabrication time of the bimorph.

Additionally, since the bimorph is formed as a monolithic single plate,the bimorph is apt to deform in an undesirable direction. This resultsin a shortcoming in which the bimorph twists and does not deformaccurately.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide aparallel plate structure having a PZT thin-film bimorph that deforms bya large degree.

Another objective of the present invention is to provide a parallelplate structure having a PZT thin-film bimorph that is optimal formass-production, can be made more compact, and resists twisting.

A further objective of the present invention is to provide a parallelplate structure having a PZT thin-film bimorph that has superiorproductivity.

A parallel plate structure, which is provided with a bimorph, accordingto the present invention, has a thin PZT thin-film formed on a first andsecond surface of a titanium substrate. Thus, the bimorph is morecompact. This, in turn, makes the entire structure more compact.Furthermore, the parallel plate structure is a duplex structure in whicha pair of bimorphs are superimposed by way of a spacer. Hence, thestructure has improved rigidity and resists twisting.

When deforming the structure, voltage having opposite polarities isapplied to adjacent electrodes on a first surface of each bimorph. As aresult, the portion of the PZT thin-film to which an electric field inthe polarity direction is applied contracts, while the portion of thePZT thin-film to which an electric field in the direction opposite thepolarity direction is applied expands.

Due to the application of the same voltage, each bimorph deforms in thesame direction. Due to the application of voltage having differentpolarities to the adjacent electrodes on each surface of each bimorph,the portions of the PZT thin-film to which the voltage of differentpolarities are applied deform in opposite directions. As a result, sincethe portions of the PZT thin-film corresponding to the adjacentelectrodes deform in opposite directions, the structure is bent in anS-shaped or reversed S-shaped manner and deformed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a cross-sectional view showing the application of voltageto a parallel plate structure, which is provided with a PZT thin-filmbimorph, according to an embodiment of the present invention;

FIG. 1(b) is a cross-sectional view showing further application ofvoltage to the structure of FIG. 1(a);

FIG. 2(a) is a cross-sectional view showing a deformed state of thestructure of FIG. 1(a);

FIG. 2(b) is a cross-sectional view showing a further deformed state ofthe structure of FIG. 1(a);

FIG. 3 is a cross-sectional view showing a substrate insert of a PCTbiomorph in accordance with the present invention;

FIG. 4 is a cross-sectional view showing the substrate of FIG. 3 of aPZT biomorph in accordance with the present invention covered by a PZTthin-film;

FIG. 5 is a cross-sectional view showing the substrate of FIG. 3 onwhich electrode films are formed;

FIG. 6 is a cross-sectional view showing a piezoelectric device formedon the substrate of FIG. 4;

FIG. 7 is a perspective view showing the piezoelectric device of FIG. 6;

FIG. 8 is an exploded perspective view showing the assembly of aparallel plate structure in accordance with the present invention;

FIG. 9 is a perspective view showing the assembled parallel platestructure of FIG. 8 in accordance with the present invention;

FIG. 10 is another perspective view showing the structure ofFIG. 8 ofFIG. 9;

FIG. 11 is another perspective view showing a structure provided with abimorph of a comparative example;

FIG. 12 is a cross-sectional view showing the structure of FIG. 11 in adeformed state;

FIG. 13 is a schematic view showing the structure of FIG. 10 in aparallel plate actuation mode; and

FIG. 14 is a schematic view showing the structure of FIG. 10 of FIG. 11in a simple bending actuation mode.

DETAILED DESCRIPTION

A parallel plate structure provided with a PZT thin-film bimorphaccording to an embodiment of the present invention will now bedescribed with reference to FIGS. 1 to 10. The thicknesses of eachmember shown in the drawings are exaggerated for illustrative purposes.

As shown in FIGS. 1(a) and (b), a parallel plate structure 1 includes apair of plate-like piezoelectric devices 2, or bimorphs, and rectangularcolumn-like insulating spacers 3, which are arranged between thepiezoelectric devices 2. The spacers 3 connect the piezoelectric devices2 at the upper and lower ends of the structure 1 and are made of aninsulating material to prevent short-circuiting between thepiezoelectric devices 2.

Each piezoelectric device 2 includes a plate-like titanium substrate 4having uniform thickness, a PZT thin-film 5 formed on the two sides ofthe titanium substrate 4, and pairs of upper and lower electrodes orelectrode films 6 formed on each PZT thin-film 5. The electrode films 6are insulated from each other. One electrode 6 film extends from one endof the associated PZT thin-film 5 substantially to the middle portion ofthe PZT thin-film 5, while the other electrode film 6 extends from theother end of the PZT thin-film 5 substantially to the middle portion.Each electrode film 6 occupies nearly half the area of the associatedPZT thin-film 5. The substrate 4 has a thickness of 20 μm, while the PZTthin-film 5 has a thickness of several tens of micrometers. Theelectrode films 6 are made of aluminum and have a thickness of severalmicrometers.

When using the structure 1 as an actuator, dc power sources B1-B4 areconnected to the structure 1 as shown in FIGS. 1(a) and (b). In FIG.1(a), the polarity direction of the PZT thin-film 5 is indicated byarrows α. The power sources B1, B2 are connected in series. The positiveterminal of each power source B1 is connected to the lower leftelectrode film 6 of the associated piezoelectric device 2. The negativeterminal of each power source B2 is connected to the lower rightelectrode film 6 of the associated piezoelectric device 2. The contactbetween the power sources B1, B2 is connected to the associated titaniumsubstrate 4.

Like the power sources B1, B2, the power sources B3, B4 are connected inseries. The positive terminal of the power source B4 is connected to theupper right electrode film 6 of the associated piezoelectric device 2,as viewed in FIG. 1(a). The negative terminal of the power source B3 isconnected to the upper left electrode film 6 of the associatedpiezoelectric device 2. The contact between the power sources B3, B4 isconnected to the associated titanium substrate 4.

The connection between the contact of the power sources B1, B2 and thetitanium substrates 4 and the connection between the contact of thepower sources B3, B4 and the titanium substrates 4 apply a uniformelectric field to the PZT thin-film 5 on each side of the titaniumsubstrates 4. However, the contacts are not required to be connected tothe substrate 4 as long as each PZT thin-film 5 has the same thickness.The power sources B1-B4 have the same voltage, which is applied to thePZT thin films 5 by the electrode films 6. Voltage having oppositepolarities is applied to the upper and lower portions of eachpiezoelectric device 2.

When the lower end (basal end) of the structure 1 is fixed to a base(not shown) and voltage is applied to the structure 1 as shown in FIG.1(a), the portion of the PZT thin-film 5 to which an electric field isapplied in the polarity direction expands in the polarity direction andcontracts in the direction perpendicular to the polarity direction. Onthe other hand, the portion of the PZT thin-film 5 to which an electricfield is applied in the direction opposite the polarity directioncontracts in the polarity direction and expands in the directionperpendicular to the polarity direction. As a result bearing in mindthat the bottom of the structure 1 is fixed, the structure 1 bends tothe left at its lower portion and bends to the right at its upperportion as shown in FIG. 2(a). Hereafter, the expansion and contractionof the PZT thin-film 5 in the direction perpendicular to the polaritydirection will be described.

When voltage having a polarity opposite that of FIG. 1(a) is applied tothe PZT thin films 5, the upper right portion and lower left portion ofeach piezoelectric device 2 expands, while the upper left portion andlower right portion of each piezoelectric device 2 contracts. As aresult, the structure 1 bends to the right at its lower portion andbends to the left at its upper portion as shown in FIG. 2(b).

In FIGS. 2(a) and (b), the electrode films 6 are not shown. Thecompressed portions 5 a of the PZT thin-films 5 are shown by the hatchlines extending downward from right to left, and the expanded portions 5b are shown by the hatch lines extending downward from left to right.

In FIG. 1(b), the polarity direction of the PZT thin-film 5 is indicatedby arrows β. As shown in the same drawing, the positive terminal of eachpower source BS is connected to the associated substrate 4. The negativeterminal of each power source B5 is connected to the electrode films 6on each side of the lower portion of the associated piezoelectric device2. The negative terminal of each power source B6 is connected to theassociated substrate 4. The positive terminal of each power source B6 isconnected to the electrode films 6 on each side of the upper portion ofthe associated piezoelectric device 2. When each electric source B5applies a positive potential to the associated PZT thin-film 5, thelower right portion of the PZT thin-film 5 expands, while the lower leftportion of the PZT thin-film 5 contracts (deforming to the left asviewed in FIG. 1(b)). When each electric source B6 applies a negativepotential to the associated PZT thin-film 5, the upper right portion ofthe PZT thin-film contracts, while the upper left portion of the PZTthin-film 5 expands. Accordingly, the structure 1 bends to the left atits lower portion and bends to the right at its upper portion as shownin FIG. 2(a). If the polarity direction of the PZT thin-films 5 is β,the same deformation amount as that of FIG. 1(a) is obtained with halfthe voltage of the power sources B1-B4 of FIG. 1(a).

Furthermore, when a voltage having a polarity opposite to that of FIG.1(b) is applied to the PZT thin-films 5, the structure 1 bends to theright at its lower portion and bends to the left at its upper portion asshown in FIG. 2(b).

The manufacturing process of the structure will now be described withreference to FIGS. 3 to 10.

As shown in FIG. 3, a titanium base material 4A is prepared. The basematerial 4A is plate-like and has a uniform thickness. The base material4A has an area equal to that of a plurality of the substrates 4 of thestructure 1. The base material 4A is first cleaned with acid or thelike. A mask M is then applied to one end (corresponding to the basalside in FIG. 1) of the base material 4A. The mask M is formed from asynthetic resin or from a metal other than titanium by employing aphysical film formation process, such as sputtering or vacuumdeposition.

Afterward, as shown in FIG. 4, a PZT thin-film 5 is formed on both sidesof the base material 4A using a hydrothermal process. The hydrothermalprocess includes two stages as described below.

First Stage

The base material 4A, raw material, which is an aqueous is solutioncontaining zirconium oxychloride (ZrOCl₂.8H₂O) and lead nitrate(Pb(NO₃)₂), and a KOH(8N) solution, which is a mineralizer, are placedin a Teflon container (not shown) and agitated. The piezoelectriccharacteristic of the PZT thin films 5 is determined by the compositionratio of lead titanate and lead zirconate. Thus, the mol ratio betweenthe zirconium oxychloride and the lead nitrate is determined inaccordance with the piezoelectric characteristic of the PZT thin films5, which are formed later.

Afterward, with the base material 4A arranged at the upper portion of apressure vessel (not shown), an aqueous solution containing zirconiumoxychloride (ZrOCl₂.8H₂O) and lead nitrate (Pb(NO₃)₂), and a KOH(8N)solution are mixed in the pressure vessel. The mixture is heated andpressurized while being agitated at a speed of 300 rpm. Pressurizingrefers to pressurizing using the vapor pressure of the heated solution.The heated and pressurized state lasts for 48 hours under a temperatureof 150° C. Consequently, PZT seed crystals (crystal nucleus) are formedon both sides of the base material 4A in a supersaturated state. Afterthe formation of the seed crystals, the base material 4A is removed fromthe pressure vessel, washed with water, and dried.

Second Stage

Subsequently, the base material 4A, on which the seed crystals areformed, raw material, which is an aqueous solution containing zirconiumoxychloride (ZrOCl₂.8H₂O) and lead nitrate (Pb(NO₃)₂), a solutioncontaining titanium tetrachloride (TiCl₄) and potassium hydroxide(KOH(4N)), which serves as a mineralizer, are placed in a Tefloncontainer (not shown) and agitated. The mol ratio between zirconiumoxychloride and lead nitrate is determined in accordance with the PZTpiezoelectric characteristic.

Then, with the base material 4A arranged at the upper portion of apressure vessel (not shown), an aqueous solution containing zirconiumoxychloride (ZrOCl₂.8H₂O) and lead nitrate (Pb(NO₃)₂), and a solutioncontaining titanium tetrachloride (TiCl4) and KOH(4N) are mixed in thepressure vessel, and heated and pressurized while being agitated at aspeed of 300 rpm. Pressurizing refers to pressurizing using the vaporpressure of the heated solution. This treatment lasts for 48 hours undera temperature of 120° C. Consequently, a PZT thin film 5 having thepredetermined thickness (in the present embodiment, several tens ofmicrometers) is formed on both sides of the base material 4A in asupersaturated state (refer to FIG. 4). After the formation of the PZTthin film 5, the base material 4A is removed from the pressure vessel,washed with water, and dried. Afterward, the mask M is removed.

As shown in FIG. 5, an electrode film 6A is then formed on each side ofthe base material 4A, which includes the PZT thin film 5, by carryingout a physical film formation process, such as sputtering or vacuumdeposition. As shown in FIGS. 6 and 7, patterning is then performed toremove unnecessary sections of the electrode films 6A in order to obtaina plurality of piezoelectric devices 2 (three in the present embodiment)from the base material 4A. As a result, in the present invention, threerows of electrode films 6, each extending in a direction indicated byarrow “A”, are formed on the PZT thin film 5 on each side of the basematerial as shown in FIG. 7. Each row includes two electrode films 6,which have the same area and the same shape. As shown in FIG. 7, theelectrode films 6 are arranged on both sides of the titanium substrate 4such that they are opposed to each other with the base material 4Aarranged in between.

As shown in FIG. 8, the unnecessary sections of the base material 4A areremoved. Subsequently, two base materials 4A, which include the PZT thinfilm 5 and the electrode films 6 are faced toward each other.Rectangular column-like insulating spacers 3, which are made fromsynthetic resin, are then arranged between the two base materials 4A.

As shown in FIG. 9, the base materials 4A and the spacers 3 are fixed toone another with an adhesive agent, which increases rigidity afterhardening, to form a parallel plate structure 1A. The structure 1A isformed by interconnecting independent structure bodies.

The structure 1A is then cut along the dotted lines between each row ofelectrode films 6 to separate the structure 1A into independent parallelplate structures 1 as shown in FIG. 10. The cutting is performed byelectric discharge machining or laser cutting.

As described above, the application of voltage having oppositepolarities to the upper and lower portions of the piezoelectric devices2 deforms the structure 1 as shown in FIGS. 2(a) or 2(b). Thisdeformation mode will hereafter be referred to as a parallel plateactuation mode.

A parallel plate structure 21, which is provided with bimorphs, is shownin FIG. 11 as a comparative example. In the drawing, like numerals areused for like elements of the structure 1 of the present embodiment.

The structure 21 of the comparative example differs from the structure 1of the present embodiment in that only one electrode film 26 is formedon each side of a titanium substrate 4. Power sources B1, B2 areconnected to the structure 21 in a series. The positive terminal of eachpower source B1 is connected to the left electrode film 26 of theassociated piezoelectric device 2. The negative terminal of each powersource B2 is connected to the right electrode film 26 of the associatedpiezoelectric device 2. The contact between the power sources B1, B2 isconnected to the associated titanium substrate 4. In the comparativeexample, the polarity direction of the PZT thin-films 5 is the same asthat shown in FIG. 1(a).

When the lower end of the structure 21 is fixed to a base (not shown)and voltage is applied as shown in FIG. 11, the portion of the PZTthin-film 5 to which an electric field is applied in the polaritydirection contracts, while the portion of the PZT thin-film 5 to whichan electric field is applied in the direction opposite the polaritydirection expands. Accordingly, the structure 21 deforms to the left asviewed in FIG. 11.

On the other hand, the application of voltage having the oppositepolarity to the piezoelectric devices 2 contracts the right PZTthin-film 5 of each piezoelectric device 2 and expands the left PZTthin-film 5 of each piezoelectric device 2. Accordingly, the structure21 deforms to the right as shown in FIG. 12. The electrode films 26 ofthe comparative example are not shown in FIG. 12. The contractedportions of the PZT thin-films 5 are shown by the hatch lines extendingdownward from right to left, and the expanded portions are shown by thehatch lines extending downward from left to right.

When the same voltage is applied to the structure 1 and the structure21, the deformation amount of the structure 1 is greater than that ofthe structure 21. The reasons for this will be described with referenceto FIGS. 13 and 14. FIG. 13 shows the deformation of the structure 1 ofthe present embodiment, while FIG. 14 shows the deformation of thestructure 21 of the comparative example.

In FIG. 14, “a” denotes deformation during application of voltage whenthe insulating spacer 3 at the free end does not exist. Attachment ofthe insulating spacer 3 interferes with the deformation of thepiezoelectric devices 2 and changes the deformation to “c” (c<a). InFIG. 13 application of voltage to, the two electrodes results in thedeformation being a/2 at the fixed end of the piezoelectric device 2.Furthermore, the free end of the piezoelectric device 2 deforms in thesame manner as the fixed side. The free end side deformation is thus a/2and the total deformation of the structure 1 is a. Accordingly, thedeformation amount of the structure 1 of the present embodiment isgreater than the deformation amount of the structure 21 when thestructures have one end fixed to a base.

In the present embodiment, the PZT thin-film 5 is thin with a thicknessof several tens of micrometers. This, decreases the size of thepiezoelectric devices 2, which in turn, decreases the size of thestructure 1.

The structure 1 of the present embodiment is a parallel plate structure,in which a pair of bimorphs, or piezoelectric devices 2, and spacers aresuperimposed. Thus, twist resistance is improved.

The fabrication method of the structure 1 of the present embodimentefficiently manufactures structures 1 having uniform quality since thehydrothermal process forms the PZT thin-film 5 and the electrode films 6simultaneously on a plurality of substrates 4. Since spacers are used tofix the bimorphs to each other, the formation of the structure 1 issimplified.

The embodiment according to the present invention may be modified asdescribed below.

The insulating spacers 3 may be replaced by spacers made of anon-insulating material, such as metal, as long as the piezoelectricdevices 2 are insulated from each other. In this case, the spacers arefixed to the piezoelectric devices 2 using other means such as welding.Furthermore, the electrode films 6 may be formed from other metals, suchas gold (AU), instead of aluminum.

The thickness of the electrode films 6, the PZT thin-films 5, and thesubstrates 4 is not limited to the above values and may be changed asrequired.

In the above embodiment, three bimorphs are obtained from the same basematerial 4A. However, two or less or four or more bimorphs may beobtained from the same material.

What is claimed is:
 1. A parallel plate structure including a pair ofparallel bimorphs (2), and a spacer (3) for connecting the bimorphs (2),wherein each bimorph (2) has a titanium substrate (4), the titaniumsubstrate (4) having a first surface and a second surface, which islocated on the opposite side of the first surface, and a PZT thin-film(5) formed on each of the first and second surfaces, wherein theparallel plate structure is characterized by: a plurality of spacedelectrodes (6) formed on each PZT thin-film (5) and extending in thesame direction.
 2. The parallel plate structure according to claim 1characterized in that the number of electrodes (6), which includes firstand second electrodes (6), on each of the first and second surfaces istwo.
 3. The parallel plate structure according to claim 2 characterizedin that the first electrode (6) extends from one end to a substantiallymiddle portion of the associated PZT thin-film (5), and the secondelectrode (6) extends from the other end to the substantially middleportion of the associated PZT thin-film (5).
 4. The parallel platestructure according to claim 1, characterized in that the PZT thin-film(5) is formed via a hydrothermal process.
 5. The parallel platestructure according to claim 1, characterized in that the spacer (3) isarranged at one end of the bimorphs (2), and a further spacer (3) isarranged at the other end of the bimorphs (2), wherein both spacers (3)are insulating bodies.
 6. A piezoelectric device, comprising: a pair ofparallel bimorphs, each bimorph including a substrate having a firstsurface and a second, opposing surface, and a PZT thin-film formed oneach of the first and second surfaces; a first spacer disposed betweenand connecting the bimorphs at a first end of the bimorphs; and at leastone electrode formed on each PZT thin-film.
 7. The piezoelectric deviceof claim 6, wherein the substrate is made of titanium.
 8. Thepiezoelectric device of claim 6, further comprising a second spacerdisposed between and connecting the bimorphs at a second end, oppositethe first end, of the bimorphs.
 9. The piezoelectric device of claim 6,wherein the spacers are made from an insulating material.
 10. Thepiezoelectric device of claim 6, wherein two electrodes are formed oneach of the PZT thin-films.
 11. The piezoelectric device of claim 10,wherein the two electrodes extend from opposing ends of the respectivesurface on which the thin-film is formed toward a center part of thesurface.
 12. The piezoelectric device of claim 11, wherein theelectrodes are made of aluminum.